Killie will welcome Neil Lennon's champions-elect to rugby Park and will be looking for another victory as they try to avoid the relegation play-off.
Two quickfire goals from top scorer Kris Boyd at the start of the second half turned the match in the home side's favour, and Johnston revealed that a change of tactics at the break played a big part in swinging the match their way.
Yet while delighted with the result, he also noted the win could have been a lot more straightforward had his team's defending been better.
"It was a really good second half," said Johnston. "We changed the system a wee bit to what we had played first half, got closer to people and got Michael (Gardyne) a wee bit higher up the pitch as well, and luckily enough it paid off.
"I think there were a lot of good performances, but we certainly make life hard for ourselves with the goals we lose.
"It's a big lift, obviously it's a big boost, but we know there's a lot to work on in terms of the goals we're conceding.
"We've got to tighten up at the back. There are a lot of hard games coming up and you can't afford to lose goals like that against top-quality opposition.
"But there are a lot of positives to take out of the performance and the result."
Hearts manager Gary Locke said: "Last week we got beaten by Motherwell and the performance wasn't good enough, but it was certainly a lot better on Saturday."